The
basic structure of these implants is shown on fig. 2. The image is captured
by a camera integrated into eyeglass frames, which follows the eye movements.
Then the image is transmitted through a transcutaneous link that stimulates
the microelectrodes implanted in the visual cortex and results into
the creation of an image (Troyk, P., et al. 2003).

The earliest experiments with
stimulation of the visual cortex were done by Brindley and Dobelle in
the late sixties and early seventies. They stimulated the visual cortex
by placing electrodes over its surface, because this area was supposed
to process the visual signals from the eyes (fig.3). Using this method
they were able to evoke phosphenes, but it was observed that multiple
phosphenes were created by one and the same electrode and that the created
phosphenes were inconsistent. Also large electrodes were used with currents
of 1–3 mA and the electrodes were spaced 3mm apart. Dobelle and his
colleagues designed a prosthesis that allowed a blind person to recognize
6-inch characters at 5 feet, which is approximately 20/1200 visual acuity
(Margalit, Eyal, et al. 2002). Dobelle also concluded that the brightness
of a phosphene is a logarithmic function of the current amplitude of
the stimulating devices. The above-mentioned experiments were not able
to provide information on how the electrical stimulation on the visual
cortex could be used to communicate images to the brain. But they confirmed
that electrical stimulation of the visual cortex could create visual
perceptions (Margalit, Eyal, et al. 2002).

The first studies with Intracortical
Implants were done by the National Institutes of Health in the early seventies.
They implanted microelectrodes directly in the visual cortex. The tip
sizes of the microelectrodes were close to the sizes of the neurons,
in this way the control of the neuron function could be achieved by
more selective stimulation. Using this kind of implant, arrays of smaller
electrodes were created, stimulating a smaller surface area on the visual
cortex and using lower current thresholds. Thus, predictable forms of
phosphenes were observed and the interactions between the different
phosphenes were reduced (Troyk, P., et al. 2003). One of the most known
Intracortical Implant, called Utah electrode array (UEA) Fig.4, was
developed by the University of Utah. They used silicon, which is highly
biocompatible, as electrode material. In UEA, large number of 1.5mm
long electrodes was separated from each other by 0.4mm space. Their
tips (fig. 5), 80-100 microns in diameter at their basis, were covered
with platinum. One of the most important design considerations was that
the array had to be very thin since it had to stay on the surface of
the brain and not interact with the skull. Because of its smaller size,
a special surgical techniques and tools had to be developed that facilitate
the implantation of the intracortical implant in the visual cortex.
Thus, a pneumatically activated insertion tool was created, fig. 6.
It was able to insert the array into the cortical tissue in 200 µs
and not damage the tissue (Normann, R. A., et al. 1999). However, the
technology is still unable to create comprehensive visual pictures in
a blind person. Dealing with the visual cortex is not a simple thing;
the special organization is very complex at cortical level. Two adjacent
neurons in the cortical tissue do not necessarily map two adjacent areas
in space. Thus pattern stimulation may not evoke pattern perception.
Also the neurons in the visual cortex control color, orientation, direction,
and depth of the visual perception. Because of that the electrode stimulation
pattern that creates a comprehensive visual pictures in the visual cortex
is hard be mapped (Normann, R. A., et al. 1999).